Anhydrous or monohydrate?

markbau · Jul 16, 2019

I have some Sodium carbonate, only problem is don't know if its Anhydrous or monohydrate. I've looked at old threads and understand the difference but I want to know, can you tell just by looking at it which one it is? The one I have looks a bit like clothes washing powder and clumps a little, although shaking the container easily breaks them up. It would not "pour" like say white table sugar would.

koraks · Jul 16, 2019

Likely a hydrate, but even then it can still be a mixture.
I once looked for a reliable way to determine which hydrate I had using fairly standard home equipment and arrived at the conclusion that measuring solubility would get me close enough.
To determine if you have a hydrate or anhydrous, you can measure a certain volume, then bake it for a while in an oven and then measure again to see if it has lost weight. All the hydrates will shed their water upon heating above boiling point.
On a sidenote, I use washing soda, which around here is virtually always the decahydrate, for pretty much all my photo chemistry and it is entirely satisfactory.

RauschenOderKorn · Jul 16, 2019

To find out which one you have, you can simply weigh it. The specific densities are rather characteristic:

2.54 kg/l anhydrous
2.25 kg/l monohydrate
1.46 kg/l Decahydrate

You could also dissolve a controlled amount in destilled water and measure the ph and determine the amount sodiumcarbonate contained..

What are you planning to use it for? For some aplications like print developers, the exact amount is not really important.

koraks · Jul 16, 2019

RauschenOderKorn said:
To find out which one you have, you can simply weigh it.

Have you actually tried that? How do you keep the volume controlled given that there's always some air in-between the particles/crystals/flakes etc.? To be honest, I don't see how this would reliably distinguish between anhydrous and monohydrate. Perhaps it works to determine if you have decahydrate.

Alan9940 · Jul 16, 2019

koraks said:
Have you actually tried that? How do you keep the volume controlled given that there's always some air in-between the particles/crystals/flakes etc.? To be honest, I don't see how this would reliably distinguish between anhydrous and monohydrate. Perhaps it works to determine if you have decahydrate.

That and doesn't an anhydrous powder absorb moisture over time?

RauschenOderKorn · Jul 16, 2019

koraks said:
Have you actually tried that? How do you keep the volume controlled given that there's always some air in-between the particles/crystals/flakes etc.? To be honest, I don't see how this would reliably distinguish between anhydrous and monohydrate. Perhaps it works to determine if you have decahydrate.

No, I have not tried, but I will.

My basic assumption is that if you don´t know what you have, probably you got a mix. Furthermore my second assumption is, that the exact dosing of sodiumcarbonate for most photographic purposes is not important (i.e. anhydrous vs. monohydrate does not matter, but decahydrate would matter).

koraks · Jul 16, 2019

Alan9940 said:
That and doesn't an anhydrous powder absorb moisture over time?

Anhydrous tends not to remain anhydrous, indeed. I think the hydrates are stable.

RauschenOderKorn said:
No, I have not tried, but I will.

My basic assumption is that if you don´t know what you have, probably you got a mix. Furthermore my second assumption is, that the exact dosing of sodiumcarbonate for most photographic purposes is not important (i.e. anhydrous vs. monohydrate does not matter, but decahydrate would matter).

Let us know if the method works; it would be convenient.
I agree that for most purposes the amount isn't very crucial, although I also use sodium carbonate in ECN2 developer and other film developers (eg instant mytol) and in that case it may be more important to stick to the right concentration. However, I do control pH of the actual developer, so that should keep variation at a minimum.

markbau · Jul 16, 2019

RauschenOderKorn said:
To find out which one you have, you can simply weigh it. The specific densities are rather characteristic:

2.54 kg/l anhydrous

2.25 kg/l monohydrate

1.46 kg/l Decahydrate

You could also dissolve a controlled amount in destilled water and measure the ph and determine the amount sodiumcarbonate contained..

What are you planning to use it for? For some aplications like print developers, the exact amount is not really important.

I'm using it for a print developer, it calls for 37.5g anhydrous or 45g monohydrate. Maybe I'll just split the difference and use 40g of what I have?

koraks · Jul 16, 2019

Well, at least then you'll have 100% certainty of not having the correct amount
Since it's a print developer and we generally develop prints to completion, I'd go with the higher amount. If it's anhydrous, you'll just have some excess carbonate and a slightly higher pH. It likely won't hurt.
Odds are that you have the monohydrate BTW. Anhydrous is a little less common.

Alan9940 · Jul 16, 2019

markbau said:
I'm using it for a print developer, it calls for 37.5g anhydrous or 45g monohydrate. Maybe I'll just split the difference and use 40g of what I have?

To convert an anhydrous amount to monohydrate, simply multiply the prior weight by 1.17; approx 43.8g in monohydrate form.

Rudeofus · Jul 17, 2019

The monohydrate has all kinds of advantages, at least that's what Kodak US wanted everyone to believe. Fact is, the monohydrate never got much traction outside the US, in Europe it's "anhydrous for chemists, and decahydrate for cleaners". From what I hear, even Kodak Harrow always used anhydrous.

So one way of finding out would be finding out, where the Sodium Carbonate is from, and if the source is likely influenced by Kodak or not.

Alan Johnson · Jul 17, 2019

In the UK it always seems to be decahydrate. Sometimes it is marked on the packet, sometimes it is on the manufacturers website and sometimes a phone call to the manufactures technical department will get that response.The decahydrate has to be kept in a sealed pack as it loses its water of crystallisation if left in the open.

RauschenOderKorn · Jul 17, 2019

markbau said:
I'm using it for a print developer, it calls for 37.5g anhydrous or 45g monohydrate. Maybe I'll just split the difference and use 40g of what I have?

The amounts of anhydrous will give the following pH-values if dissolved in 1l of distilled water:
(source: http://www.periodensystem-online.de/index.php?id=calc&form=phwert )

37.5 g/l -> pH 11,97
40 g/l -> pH 11,98
45 g/l -> pH 12.01

This difference is well within the margin of error of most ph-measurement devices.

If you use 37,5 g/l, your developer last a little shorter compared to the 45 g/l. But you will only notice a difference if you use your developer to exhaustion. If you lose it to oxidization (storage) anyway, you will not be able to notice any difference.

RauschenOderKorn · Jul 17, 2019

koraks said:
I also use sodium carbonate in ECN2 developer and other film developers

For film developers, I would check if an increased amount will impact in a significant change in ph. If not, I´d always go with the higher amount. You do not want to run out of OH- during film development.

koraks · Jul 17, 2019

Indeed, and since pH is critical for color development, I measure it and adjust as necessary. Because I generally don't reuse the color developer, buffering capacity is a little less important. I also use the decahydrate so I use the higher amount in any case - but then again I've determined with certainty that I do in fact have the decahydrate, so it's not a matter of guesswork.

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